Here, \(\alpha \)  runs over a basis for the space of polarisations \(V\)  of the four fields and \(I\)  runs over the space for the space of four-point tensor structures \(W\) . If \(d>3\)  we have in general \(\dim W<\dim V\) . Detailed discussions of tensor structures can be found in [1]}, [2]}, [3]}, [4]}, [5]}. We will summarise some of the main points that will be important in the later subsections. For concreteness, let us focus on bosonic theories.
